There is a need for a robust, accurate, and built-in weighing system for highways (and other applications)—not only to determine vehicle weights for freight accuracy and weight compliance—but also for security reasons. Every roadway can be a path for the transport of illegal cargo. In one example of transport of hazardous materials, chemical, biological, or radiation monitors may be used to detect such materials. But many of those materials may be shielded from detection using various linings, like a lead lining or other heavy absorbing materials such as concrete. In that situation, a vehicle with an adequate suspension could carry a hazardous payload undetected by chemical, biological, or radiation monitors. That vehicle, however, will exceed statistical weight limits and could be identified by a weigh-in-motion sensor built into the highway. If a network of weigh-in-motion sensors were constructed, detection and surveillance of suspect vehicles could be readily accomplished. Ideally, that network should be invisible, highly verifiably accurate, able to obtain weight at high vehicular speeds, cost effective, potentially able to be wireless, to be robust and exhibit a long life.
Current technology for weighing vehicles employs a scale supported independently from the roadbed. The scale may use one or more types of sensors to measure weight such as strain gauges, springs, pressure sensors, and deflectometers. These scales require a vehicle to be stationary or nearly stationary for weight measurement, a measurement platform concentrating the vehicle load to a sensor(s) (spring or strain gauge or other device), and a readout device such as a deflection scale or an electronic readout.
There is a need for an accurate, robust, reliable, and easily-implemented method to weigh moving vehicles. The technology described below meets these and other needs using acoustic sensors attached to or embedded in the load bearing medium, e.g., the road. Although piezoelectric sensors are embedded in a highway, they are less than optimal. Because a piezoelectric sensor measures strain, vehicle weight would have to be inferred based on how the vehicle weight is conducted to the piezoelectric sensor. This is problematic because the vehicle may move over the device at unpredictable distances. Also, piezoelectric devices are usually covered with a layer of material whose stiffness can change in time and can wear altering the calibration and therefore accuracy of the piezoelectric sensor output. Piezowires are fragile and can be damaged in use. There are similar problems with burying other types of sensors, like linear voltage differential transducers (LVDTs), capacitive plates, inductive gap systems, optical fibers and other discrete sensors, in the roadbed.